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What's the 'Shannon Limit' and Why Is It Important to Networking?

5G is coming, and it’s predicted to bring with it…well, nobody’s really sure EXACTLY what, but it’s expected to surpass existing 4G LTE, LTE-Advanced, and LTE-Advanced Pro mobile networks in speed and low latency many times over, leading to the development of more and larger content.

Here’s the thing, however: more than 95 percent of the world’s intercontinental electronic communications traffic is carried by submarine networks (literally, huge cables along the ocean floor), and this traffic is subject to a rule of physics called the “Shannon limit.”

What’s The Shannon Limit?

The Shannon Limit is named for a 1948 research paper by MIT professor Claude Shannon, who first mapped out the relationship between bandwidth and noise. Bandwidth, of course, is the range of electronic, optical or electromagnetic frequencies that can be used to transmit a signal. “Noise” is anything that can disturb that signal.

“Given a channel with particular bandwidth and noise characteristics, Shannon showed how to calculate the maximum rate at which data can be sent over it with zero error,” wrote Larry Hardesty of MIT News. “He called that rate the channel capacity, but today, it’s just as often called the Shannon limit.”

The more noise there is, the rule dictates, the more errors will creep into the channel. To compensate for these errors, network developers must add redundancy: extra information added to a message so that errors can be corrected if they occur. The more error-correcting code that must be added, of course, the lower transmission rates go. It’s simply not a rule that can be “5Ged” around anymore than an airplane design can change the behavior of gravity.

“Although the incredible wizardry of coherent modem technology continues unabated, unfortunately, we can’t break the laws of physics and are faced with a real limit to the maximum information-carrying capability of a submarine optical fiber,” wrote Brian Lavallée, submarine network expert for Ciena.

What’s The Path Forward?

So, given the fact that 95 percent of global networking traffic is carried by submarine cables, and the unyielding rules of physics, what’s the next step going to be? According to Lavallée, we have two choices: deploy more cables, and/or develop new submarine cable designs with more fiber pairs.

The “deploying more cables” option is already happening, but it’s not something that can occur overnight. Instead, companies like Ciena are planning to use Spatial Division Multiplexing (SDM) technology to add more fiber pairs to future submarine cables (as many as 32 compared to the current 4 to 8).

“The basic tenet of SDM is to operate at a lower optical signal-to-noise ratio (OSNR) and lower total output power (TOP) per fiber pair, for better power efficiency, and use the saved power to power more fiber pairs,” wrote Lavallée.

The result, he says, a lower per fiber pair capacity, but a higher overall submarine cable capacity.

Web-scale Networking

The idea of Web-scale IT is more than just another 'hot' buzzword or problematic disruption. What started with data center operators has become mainstream thinking in large enterprises, and it's now driving changes in service provider operations, as well. Web-scale tools that allow application development to move quickly have also created some challenges for service provider networks.